1. Field of the Invention
The present invention relates to a liquid crystal display, and more particularly to a liquid crystal display that is adaptive for automatically adjusting a swing of a gamma reference voltage which is used for determining a gray scale level of a frame that is driven with a doubled frame frequency in real time, and a driving method thereof.
2. Description of the Related Art
Generally, a liquid crystal display controls light transmittance of liquid crystal cells in accordance with video signals to thereby display a picture. An active matrix type of liquid crystal display having a switching device provided for each liquid crystal cell is advantageous for an implementation of moving picture because it permits an active control of the switching device. The switching device used for the active matrix liquid crystal display mainly employs a thin film transistor (hereinafter, referred to as “TFT”) as shown in FIG. 1.
Referring to FIG. 1, the liquid crystal display of the active matrix type converts a digital input data into an analog data voltage on the basis of a gamma reference voltage to supply it to a data line DL and, at the same time supply a scanning pulse to a gate line GL, thereby charging a liquid crystal cell Clc.
A gate electrode of the TFT is connected to the gate line GL, a source electrode is connected to the data line DL, and a drain electrode of the TFT is connected to a pixel electrode of the liquid crystal cell Clc and one end electrode of a storage capacitor Cst.
A common electrode of the liquid crystal cell Clc is supplied with a common voltage Vcom.
When the TFT is turned-on, the storage capacitor Cst charges a data voltage applied from the data line DL to constantly maintain a voltage of the liquid crystal cell Clc.
If the gate pulse is applied to the gate line GL, the TFT is turned-on to define a channel between the source electrode and the drain electrode, thereby supplying a voltage on the data line DL to the pixel electrode of the liquid crystal cell Clc. In this case, liquid crystal molecules of the liquid crystal cell Clc are arranged by an electric field between the pixel electrode and the common electrode to modulate an incident light.
A configuration of a liquid crystal display of a related art including pixels which have such a structure is the same as shown in FIG. 2.
FIG. 2 is a block diagram showing a configuration of a liquid crystal display of the related art.
Referring to FIG. 2, the liquid crystal display device 100 of the related art includes a liquid crystal display panel 110, a data driver 120, a gate driver 130, a gamma reference voltage generator 140, a backlight assembly 150, an inverter 160, a common voltage generator 170, a gate driving voltage generator 180, and a timing controller 190. Herein, the data driver 120 supplies a data to the data lines DL1 to DLm of the liquid crystal display panel 110. The gate driver 130 supplies a scanning pulse to the gate lines GL1 to GLn of the liquid crystal display panel 110. The gamma reference voltage generator 140 generates a gamma reference voltage to supply it to the data driver 120. The backlight assembly 150 irradiates a light onto the liquid crystal display panel 110. The inverter 160 applies an AC voltage and a current to the backlight assembly 150. The common voltage generator 170 generates a common voltage Vcom to supply it to the common electrode of the liquid crystal cell Clc of the liquid crystal display panel 110. The gate driving voltage generator 180 generates a gate high voltage VGH and a gate low voltage VGL to supply them to the gate driver 130. The timing controller 190 controls the data driver 120 and the gate driver 130.
The liquid crystal display panel 110 has a liquid crystal dropped between two glass substrates. On the lower glass substrate of the liquid crystal display panel 110, the data lines DL1 to DLm and the gate lines GL1 to GLn perpendicularly cross each other. Each intersection between the data lines DL1 to DLm and the gate lines GL1 to GLn is provided with the TFT. The TFT supplies a data on the data lines DL1 to DLm to the liquid crystal cell Clc in response to the scanning pulse. The gate electrode of the TFT is connected to the gate lines GL1 to GLn while the source electrode thereof is connected to the data line DL1 to DLm. Further, the drain electrode of the TFT is connected to the pixel electrode of the liquid crystal cell Clc and to the storage capacitor Cst.
The TFT is turned-on in response to the scanning pulse which is applied, via gate lines GL1 to GLn, to the gate terminal. Upon turning-on of the TFT, a video data on the data line DL1 to DLm is supplied to the pixel electrode of the liquid crystal cell Clc.
The data driver 120 supplies a data to the data lines DL1 to DLm in response to a data driving control signal DDC which is supplied from the timing controller 190. Further, the data driving circuit 120 samples and latches digital video data RGB which are supplied from the timing controller 190, and then converts them into an analog data voltage that is capable of realizing a gray scale at the liquid crystal cell Clc of the liquid crystal display panel 110 on the basis of a gamma reference voltage which is supplied from the gamma reference voltage generator 140 to supply it to the data lines DL1 to DLm.
The gate driver 130 sequentially generates a scanning pulse in response to a gate driving control signal GDC and a gate shift clock GSC which are supplied from the timing controller 190 to supply them to the gate lines GL1 to GLn. In this case, the gate driver 130 determines a high level voltage and a low level voltage of the scanning pulse in accordance with the gate high voltage VGH and the gate low voltage VGL which are supplied from the gate driving voltage generator 180.
The gamma reference voltage generator 140 receives a high-level power voltage VDD to generate a positive gamma reference voltage and a negative gamma reference voltage to output them to the data driver 120.
The backlight assembly 150 is provided at the rear side of the liquid crystal display panel 110, and is radiated by an AC voltage and a current which are supplied from the inverter 160 to irradiate a light onto each pixel of the liquid crystal display panel 110.
The inverter 160 converts a square wave signal generated at the interior thereof into a triangular wave signal, and then compares the triangular wave signal with a direct current power voltage VCC supplied from the system to generate a burst dimming signal proportional to the result. If the burst dimming signal is generated, then a driving integrated circuit IC (not shown) within the inverter 160 controls a generation of AC voltage and current supplied to the backlight assembly 150 in accordance with the burst dimming signal.
The common voltage generator 170 receives a high-level power voltage VDD to generate a common voltage Vcom, and supplies it to the common electrode of the liquid crystal cells Clc provided at each pixel of the liquid crystal display panel 110.
The gate driving voltage generator 180 is supplied with a high-level power voltage VDD to generate the gate high voltage VGH and the gate low voltage VGL, and supplies them to the gate driver 130. Herein, the gate driving voltage generator 180 generates a gate high voltage VGH more than a threshold voltage of the TFT provided at each pixel of the liquid crystal display panel 110 and a gate low voltage VGL less then the threshold voltage of the TFT. The gate high voltage VGH and the gate low voltage VGL generated in this manner are used for determining a high level voltage and a low level voltage of the scanning pulse generated by the gate driver 130, respectively.
The timing controller 190 supplies digital video data RGB which are supplied from a system such as a TV set or a computer monitor, etc to the data driver 120. Furthermore, the timing controller 190 generates a data driving control signal DCC and a gate driving control signal GDC using horizontal/vertical synchronization signals H and V from a system in response to a clock signal CLK from a system to supply them to the data driver 120 and the gate driver 130, respectively. Herein, the data driving control signal DDC includes a source shift clock SSC, a source start pulse SSP, a polarity control signal POL, and a source output enable signal SOE, etc. The gate driving control signal GDC includes a gate start pulse GSP and a gate output enable signal GOE, etc.
In general, the liquid crystal display 100 of the related art having such a configuration and a function is driven with a frame frequency of 60 Hz. However, in order to improve a motion blur, a technique that a liquid crystal display is driven with a frame frequency of 120 Hz has been developed recently.
When the liquid crystal display of the related art is driven with a frame frequency of 120 Hz, the gamma reference voltage generator 140 generates a plurality of gamma reference voltages GMA1 to GMA14 that have a different level to supply them to the data driver 120 as shown in FIG. 3. Herein, a plurality of gamma reference voltages GMA1 to GMA14 which are being supplied to a doubled odd numbered frame and a doubled even numbered frame maintain a constant level without a swing as shown in FIG. 3.
Since levels of the gamma reference voltages GMA1 to GMA14 are constantly maintained, a doubled odd numbered frame and a doubled even numbered frame of which a gray scale is realized proportional to levels of the gamma reference voltages GMA1 to GMA14 have a gray scale converting characteristics as shown in FIG. 4.
FIG. 4 is a diagram showing a gray scale converting characteristics of the liquid crystal display of the related art which is driven with a doubled frame frequency. In other words, FIG. 4 shows a gray scale converting characteristics in a normally black mode which displays a black screen in a normally state that a data voltage is not applied to the liquid crystal display panel 110 and shows a gray scale converting characteristics in a normally white mode which displays a white screen in a normally state that a data voltage is not applied to the liquid crystal display panel 110.
Referring to FIG. 4, levels of the gamma reference voltages GMA1 to GMA7 corresponding to a high gray scale and levels of the gamma reference voltages GMA8 to GMA14 corresponding to a low gray scale are changed in symmetrical to each other, and gray scales which are realized in proportional to the levels are changed in symmetrical to each other on the basis of an intermediate gray scale at a doubled odd numbered frame and a doubled even numbered frame.
As described above, the liquid crystal display of the related art does not swing the gamma reference voltages GMA1 to GMA14, and doubles only a frame frequency upon doubling and driving of a frame. As a result, in the liquid crystal display of the related art, a flicker is generated by an alternate realization of a high gray scale and a low gray scale.